In recent years, in addition to mobile communication systems, in particular, communication systems using mobile phones or mobile terminals, systems for dealing with information communication such as Bluetooth is being rapidly put to practical use. Particularly in Bluetooth, because a communication module constituting a communication system is added to a conventional device, the communication module is requested to be compact, light in weight, and high in performance in order to maintain the original design of the device. On the other hand, in case of card-shaped communication modules and the like used for data communication using mobile phones or personal computers, there is a tendency that they are made thinner and smaller.
In general, as shown in FIG. 10, such a communication module is formed in such a manner that an antenna element 205 in which a radiation electrode 203 is formed on a surface of a dielectric substrate 202, is mounted at a predetermined position on the upper face of a module substrate 206 on which a signal processing circuit is formed.
In this communication module, in order to meet the above-described request of reducing the size of the communication module, in the module substrate 206, the integration of a high frequency circuit unit IC, a baseband unit IC, and so on, for processing signals to be transmitted to the antenna element 205 and signals received from the antenna element 205, has been improved, and the circuit substrate has been made smaller and thinner by forming passive elements in a laminated substrate, which elements were conventionally arranged on the substrate.
On the other hand, for the antenna element, there are various forms such as a dipole antenna, a monopole antenna, an inverted F-type antenna, and a microstrip antenna. Of them, there is a tendency of adopting the monopole antenna or microstrip antenna, in which a radiation electrode made of conductive material is formed on a dielectric substrate. This is because that they are antennas utilizing radiation loss of an open type resonance circuit; they are low in profile and light in weight and suitable for reduction in size; they are easy to manufacture; and so on. The monopole antenna is suitable for operation in broader band frequencies than a microstrip antenna. Such antennas are disclosed in JP-A-9-153734, JP-A-11-112221, etc.
As described above, such a dielectric antenna module is requested to have the optimum construction that enable to obtain sufficient transmission/reception characteristics in the combination of the antenna element 205 and the module substrate 206, and further to be made compact, light in weight, and high in performance. In the conventional dielectric antenna module as shown in FIG. 10, however, the antenna element 205, an IC chip 207, etc., are disposed on the upper face of the module substrate 206. Thus, in order to meet the request of reducing the thickness of the module, the thickness of the antenna element 205 must be set to a value corresponding to the height of the peripheral parts such as IC chips so that the antenna element 205 does not protrude upward. However, in the antenna element 205, differently from the other parts, a free space provided in the vicinity of the radiation electrode 203 for stabilizing the transmission/reception characteristics. Thus, in practice, there is a restriction that the height of the antenna element must be somewhat lower than the peripheral parts.
However, it is known in general that reducing the thickness of such an antenna element may bring about a reduction in band width (an increase in unloaded Q) and thus a reduction in radiation efficiency. The thickness of the dielectric antenna element mounted on such a module is in general very small as 1/10 or less of the electric signal wavelength. Therefore, the reduction rates of the band width and radiation efficiency by reducing the thickness of the antenna element are high. Thus, in case that an antenna for broad band communication such as Bluetooth is reduced in size, designing for broad band had to be done at the sacrifice of gain or radiation efficiency. Because the thicker antenna element brings about the less sacrifice of gain or radiation efficiency, it is advantageous on the point of broad band. Thus, in case of attaching great importance to the antenna performance, the antenna element may be mounted as the tallest part on the module substrate.
In addition, because electric signal loss occurs in the connecting portion between the antenna element unit and the module substrate unit and in the transmission line, it is desired that the number of connecting portions is reduced as much as possible and the transmission line is shortened as much as possible. In case that the antenna element is adjacent to the module substrate to shorten the transmission line, the interference may occur between the respective electromagnetic components generated therein. There is a necessity of suppressing the interference.